1. Technical Field
The present invention relates to a liquid crystal display panel with protective circuits that prevent breakage of active elements due to electrostatic during the manufacture process, and more particularly to a liquid crystal display panel with protective circuits that has low power consumption and good display quality.
2. Related Art
There is a simple matrix type of liquid crystal display panel and an active matrix type. However, in the simple matrix type items with the progression to high-definition liquid crystal display panels there has arisen a deterioration of the picture contrast, due to the occurrence of crosstalk. To prevent this, widespread use is now made of active matrix type liquid crystal display panels that are provided with a switching element such as a thin film transistor (TFT) or thin film diode in each individual pixel.
Active matrix type liquid crystal display panels with such structure have come to be manufactured in sizes ranging from small-sized items for mobile telephones to large items of diagonal size 50 inches (approximately 125 cm) to 60 inches (approximately 152 cm) or so. However, if electrostatic penetrates into the display area of liquid crystal display panels during the manufacturing process, resulting in short-circuiting, etc., of the wiring, there will be danger that line defects and point defects will occur in the display screen of the finished liquid crystal display panels, which will have large impacts on the production yield of liquid crystal display panels.
Electrostatic will occur simply through contact with some other thing, either in the manufacturing process or during conveying of the panels. For instance, in the course of processing of a liquid crystal display panel, which involves placing the array substrate, with TFTs and so forth formed thereon, on an exposure stage and implementing specific processes including exposure to light, electrostatic charging will occur when the array substrate is positioned on the exposure stage, because of friction between the array substrate, which is constituted of glass, and the exposure stage. Also, peel-off charging will occur when the array substrate is moved off the exposure stage by the conveying means after the light exposure has finished.
As the electrostatic builds up, electrostatic damage such as breakage of the TFTs formed on the substrate, and short-circuits caused between wiring lines, will occur. Electrostatic is most liable to be generated by the friction during rubbing of the alignment layer. Particularly in small- and medium-sized equipment, electrostatic faults will be more liable to occur than previously as the progression to higher definitions advances. Thus, in the field of liquid crystal display panel manufacturing technology it is an urgent task to prevent the occurrence of display defects due to electrostatic.
One measure for preventing electrostatic damage to the array substrate that has been taken in the related art is to provide around the outer periphery of the wiring lines of the liquid crystal display an outer shorting ring, which is subsequently severed from the wiring lines, and to connect the various wiring lines to the outer shorting ring so that electrostatic is dispersed among the wiring lines, thus alleviating the voltage that is generated between scan lines and picture lines.
However, the outer shorting ring is severed during the substrate cutting process, and no measure for preventing electrostatic damage to the array substrate of the liquid crystal display panel after severing of the outer shorting ring is provided. Thus, antistatic measures for after severing of the outer shorting ring are a problem. Accordingly, a liquid crystal display panel 50 shown in FIG. 7 has been proposed (see JP-A-63-10558), in which, as an antistatic measure for after severing of the outer shorting ring, an inner shorting ring E is provided and scan lines X and picture lines Y are each connected to the inner shorting line E via electrostatic protection circuits constituted of protective transistors TFT1 to TFT4.
A liquid crystal display panel 60 has also been proposed (see JP-A-10-303431), in which, as shown in FIG. 8, the scan lines X and picture lines Y are each connected to separate shorting wires 63, 64 via separate electrostatic protection circuits 61, 62, and are each further connected to separate shorting bars 65, 66, with a lowest level voltage Vg1 of the voltage applied to the scan lines being applied to the scan line shorting wire 63, and a stable, common voltage Vcom being applied to the picture line shorting wire 64.
However, with the invention disclosed in JP-A-63-10558 there has been a problem of high power consumption, because the wiring lines that are connected to the ground line are also, connected to the inner shorting ring E via the protective transistors TFT1 to TFT4, due to the electrostatic damage countermeasure, and therefore current unavoidably flows between the ground line and each scan line X and picture line Y.
Also, with the invention disclosed in JP-A-10-303431, the scan lines X and picture lines Y are each doubly connected—on the inner side to the shorting wires 63, 64 via the electrostatic prevention circuits 61, 62 and on the outer side to the shorting bars 65, 66—but there is still the problem that after the outer shorting bars 65, 66 have been severed, the power consumption is high due to leakage current.
Moreover when, as another electrostatic damage countermeasure, the present inventors formed the inner shorting ring, connected the scan lines and picture lines thereto separately via protective circuits, and connected such to a common potential line formed on the array side of the array substrate, the same problem of high power consumption occurred as in the liquid crystal display panel 50 or 60 of the related art described above. Such common potential line potential had the same potential as that applied to the color filter substrate's common electrodes, and was connected to the common electrodes in the four corners of the substrate.
The cause of the high power consumption is, according to the findings of investigations by the present inventors, as follows. Each of the wiring lines connected to the common electrodes is also connected to the inner shorting ring via transistors, as an electrostatic damage countermeasure. But if the wires are connected to the common potential line via transistors, as an electrostatic damage countermeasure, then, as is evident from the voltage waveform diagram shown in FIG. 9 for when the liquid crystal display panel is driven, a voltage Voff will be permanently applied to the scan lines even when scanning is not performed, while when scanning is performed, a voltage Von will be applied thereto, so that there will be large potential difference between the voltages applied to the common potential line and the scan lines, with the result that current will flow between the common potential line and the scan lines, in particular. This current results in the high power consumption.
Accordingly the present inventors pursued many and various investigations in order to solve the foregoing problem of high power consumption in liquid crystal display panels provided with electrostatic damage prevention circuits, and as a result discovered that by bundling with a shorting wire the scan lines and the picture lines, respectively, which are connected to transistors as an electrostatic damage countermeasure, and then connecting to the inner shorting ring, via transistors, each line bundle bundled by the shorting wires, it is possible to curb the leakage current that arises between these lines and the inner shorting ring, and thereby to provide a liquid crystal display panel with low power consumption, as has already been disclosed in JP-A-2005-275004.
A liquid crystal display panel 10B that is disclosed in JP-A-2005-275004 will now be described using FIG. 10. In this liquid crystal display panel 10B, each area enclosed by the scan lines 11 and picture lines 12, which are arrayed as a matrix, is a pixel area, and in each pixel area there is formed, say, a TFT 13 or thin film diode serving as switching element, and a pixel electrode 14. In FIG. 10 the gate electrode, drain electrode and source electrode of the TFTs 13, which are the switching elements, are connected to a scan line 11, picture line 12 and pixel electrode 14 respectively. With such configuration, when scan signals are supplied to the scan lines 11, the TFTs 13 are put into the ON state, and picture signals from the picture lines 12 are supplied to the pixel electrodes 14 via the TFTs 13, which are in the ON state.
In the liquid crystal display panel 10B, electrostatic protection circuits 151, 152 are connected to each scan line 11 and each picture line 12, then the scan lines 11 and picture lines 12 are bundled with shorting lines 161 and 162 respectively, and the scan line 11 and picture line 12 bundles bundled by the shorting lines 161 and 162 are connected to a common potential line (that includes ground) 17. Also, other electrostatic protection circuits 181 and 182 are interposed and connected between the shorting lines 161 and 162 bundling the scan lines 11 and picture lines 12 and the common potential line 17. In this case, the common potential line 17 is an electrode provided on the array substrate of the liquid crystal display panel, and at one of the array substrate's four corners is connected to a common electrode of the color filter substrate.
Further, although this liquid crystal display panel 10B represents an example where, for the first electrostatic protection circuits 151, 152 and second electrostatic protection circuits 181, 182, use is made of a pair of diodes connected in parallel, with the anode of each connected to the cathode of the other, it would alternatively be possible to use parallel circuits in which a pair of transistors, with short-circuits created between their gates and sources, are connected in opposite orientations to each other.
Even if electrostatic or other high voltage with differing positive/negative polarity is applied to the electrostatic protection circuits 151, 152, 181, 182, one of the diodes thereof, or else one of the protective transistors that turn on with positive bias or one of the protective transistors that turn on with negative bias, will be put into the ON state and the high voltage will be conducted into the common potential, so that the TFTs serving as active elements will be protected and electrostatic damage will be prevented.
Moreover, before testing of the array substrate, short circuits are normally created between the various wiring lines by the outer shorting ring (not shown in the drawing), and thus there is protection against electrostatic. After testing of the array substrate, when the liquid crystal display panel has been severed from the outer shorting ring, occurrence of variation or the like in Vth, the threshold level for TFT 13 voltage due to electrostatic, will be prevented by the duplex electrostatic protection circuits 151, 152 and 181, 182 connected to the shorting lines 161, 162 and the common potential line 17.
As regards power consumption, with the scan lines 11 and picture lines 12 bundled via the first electrostatic protection circuits 151, 152, and moreover connected to the common potential line 17 via the diodes or TFTs of the second duplex electrostatic protection circuits 181, 182, current will not readily flow between each line bundle and the common potential line 17, insofar as the diodes or TFTs of the second duplex electrostatic protection circuits 181, 182 are interposed therebetween. Thanks to this, leakage current to the common potential line 17 will be curbed and a low power consumption liquid crystal display panel 10B will be obtained.
The liquid crystal display panel 10B disclosed in JP-A-2005-275004 as described above yields exceedingly outstanding advantages in terms of being strong against electrostatic damage during manufacture and having low power consumption. However, increasingly higher definition than in earlier items is being required of the small-sized liquid crystal display panels of recent years, which are typified by those used in mobile telephones, and to increase the definition of such liquid crystal display panels it is indispensable to render the various wiring line portions narrower so as to render the display aperture larger. But as the progression to higher definition has advanced in the liquid crystal display panels with the electrostatic protection circuits such as described above, flicker has been found to occur.